[5.3] Sync collections documentation

[paulofreitas]

Syncs Laravel 5.3 collections documentation (including Eloquent collections)

Added

• Added 7 missing methods:
  • average() (backports [5.4] Misc improvements to collections documentation #3228)
  • containsStrict() from [5.3] Add containsStrict method framework#14657 (since v5.3.0-RC1; backports [5.4] Document Collection::containsStrict() method #3222)
  • isNotEmpty() from [5.3] Add isNotEmpty() method to Collection class framework#16797 (since v5.3.27; backports [5.4] Document Collection::isNotEmpty() method #3221)
  • median() and mode() from [5.2] Add median and mode functions to collections framework#14305 (since v5.2.40; backports [5.4] Document Collection::median() and Collection::mode() methods #3223)
  • partition() from [5.3] Add partition collection method framework#16627 (since v5.3.27; backports ecc1751)
  • uniqueStrict() from [5.3] Add uniqueStrict method framework#14661 (since v5.3.0-RC1; backports [5.4] Document Collection::uniqueStrict() method #3224)
• Added 10 missing methods to Eloquent collections documentation; all the listed above plus:
  • mapWithKeys() (since v5.3.0)
  • pipe() (since v5.2.40)
  • split() (since v5.3.7)

Changed

• Improved avg(), contains() and unique() methods to align with 5.4 (ref. [5.4] Document Collection::containsStrict() method #3222, [5.4] Document Collection::median() and Collection::mode() methods #3223, [5.4] Document Collection::uniqueStrict() method #3224)
• Various minor corrections and additions as was made to 5.4 (ref. [5.4] Misc improvements to collections documentation #3228)

Fixed

• Fixed two minor typos on prepend() method code
• Fixed a minor typo on union() method code